Self-consistent electronic-structure calculations of the (101-bar0) surfaces of the wurtzite compounds ZnO and CdS

Ion Irradiation-Induced Coordinatively Unsaturated Zn Sites for Enhanced CO Hydrogenation

Defect engineering critically influences metal oxide catalysis, yet controlling coordinatively unsaturated metal sites remains challenging due to their inherent instability under reaction conditions. Here, we demonstrate that high-flux argon ion (Ar+) irradiation above recrystallization temperatures generated well-defined coordinatively unsaturated Zn (CUZ) sites on ZnO(101̅0) surfaces that exhibited enhanced stability and activity for CO hydrogenation. Combining low-temperature scanning probe microscopy, ambient pressure X-ray photoelectron spectroscopy, and surface-ligand infrared spectroscopy with density functional theory calculations, we identified <12̅10> step edges exposing CUZ sites as the dominant active sites. These sites facilitate hydrogen-assisted CO dissociation through a mechanism distinct from formate-mediated pathways on stoichiometric ZnO. The ion-irradiation approach effectively addressed instability of Zn species, a major problem in ZnO catalysis, enabling stable performance in syngas conversion when combined with zeolites. Our atomic scale investigation provided spectroscopic fingerprints for active sites on the ZnO catalyst and insights into the structure-activity relationships of ZnO for CO hydrogenation. Our approach for engineering thermally stable defect sites in oxide catalysts provided opportunities for rational catalyst design beyond traditional preparation methods.

CO2 and CO Capture on the ZnO Surface: A GCMC and Electronic Structure Study

The amount of polluting gases released into the atmosphere has grown drastically. Among them, it is possible to cite the release of CO2 and CO gases on a large scale as one of the products of the complete and incomplete combustion of petroleum-derived fuels. It is worth noting that the production of energy by burning fossil fuels supplies the energy demand but causes environmental damage, and several studies have addressed the reduction. One of them is using materials with the potential to capture these gases. The experimental and theoretical studies have significant contributions that promote advances in this area. Among the materials investigated, ZnO has emerged, demonstrating the considerable potential for capturing various gases, including CO2 and CO. This work used density functional theory (DFT) and Grand Canonical Monte Carlo Method (GCMC) to investigate the adsorption of CO2 and CO on the surface of Zinc oxide (ZnO) to obtain adsorption isotherms and interaction energy and the interaction nature. The results suggest that CO2 adsorption slightly changed the angle of the O-C-O to values less than 180°. For the CO, its carbon atom interacts simultaneously with Zn and O of the ZnO surface. However, CO interactions have an ionic character with a lower binding energy value than the CO2 interaction. The energies calculated using the PM6 and DFT methods generated results compatible with the experimental values. In applications involving a mixture of these two gases, the adsorption of CO2 should be favored, and there may be inhibition of the adsorption of CO for high CO2 concentrations.

Anisotropy Engineering of ZnO Nanoporous Frameworks: A Lattice Dynamics Simulation

The anisotropy engineering of nanoporous zinc oxide (ZnO) frameworks has been performed by lattice dynamics simulation. A series of zinc oxide (ZnO) nanoporous framework structures was designed by creating nanopores with different sizes and shapes. We examined the size effects of varying several features of the nanoporous framework (namely, the removal of layers of atoms, surface-area-to-volume ratio, coordination number, porosity, and density) on its mechanical properties (including bulk modulus, Young's modulus, elastic constant, and Poisson ratio) with both lattice dynamics simulations. We also found that the anisotropy of nanoporous framework can be drastically tuned by changing the shape of nanopores. The maximum anisotropy (defined by Y_max/Y_min) of the Young's modulus value increases from 1.2 for bulk ZnO to 2.5 for hexagon-prism-shaped ZnO nanoporous framework structures, with a density of 2.72 g/cm³, and, even more remarkably, to 89.8 for a diamond-prism-shape at a density of 1.72 g/cm³. Our findings suggest a new route for desirable anisotropy and mechanical property engineering with nanoporous frameworks by editing the shapes of the nanopores for the desired anisotropy.

Probing surface defects of ZnO using formaldehyde

The catalytic properties of metal oxides are often enabled by surface defects, and their characterization is thus vital to the understanding and application of metal oxide catalysts. Typically, surface defects for metal oxides show fingerprints in spectroscopic characterization. However, we found that synchrotron-radiation photoelectron spectroscopy (SRPES) is difficult to probe surface defects of ZnO. Meanwhile, CO as a probe molecule cannot be used properly to identify surface defect sites on ZnO in infrared (IR) spectroscopy. Instead, we found that formaldehyde could serve as a probe molecule, which is sensitive to surface defect sites and could titrate surface oxygen vacancies on ZnO, as evidenced in both SRPES and IR characterization. Density functional theory calculations revealed that formaldehyde dissociates to form formate species on the stoichiometric ZnO(101¯0) surface, while it dissociates to formyl species on Vo sites of the reduced ZnO(101¯0) surface instead. Furthermore, the mechanism of formaldehyde dehydrogenation on ZnO surfaces was also elucidated, while the generated hydrogen atoms are found to be stored in ZnO bulk from 423 K to 773 K, making ZnO an interesting (de)hydrogenation catalyst.

Size-dependent Young's modulus in ZnO nanowires with strong surface atomic bonds

The mechanical properties of size-dependent nanowires are important in nano-electro-mechanical systems (NEMSs), and have attracted much research interest. Characterization of the size effect of nanowires in atmosphere directly to broaden their practical application instead of just in high vacuum situations, as reported previously, is desperately needed. In this study, we systematically studied the Young's modulus of vertical ZnO nanowires in atmosphere. The diameters ranged from 48 nm to 239 nm with a resonance method using non-contact atomic force microscopy. The values of Young's modulus in atmosphere present extremely strong increasing tendency with decreasing diameter of nanowire due to stronger surface atomic bonds compared with that in vacuum. A core-shell model for nanowires is proposed to explore the Young's modulus enhancement in atmosphere, which is correlated with atoms of oxygen occurring near the nanowire surface. The modified model is more accurate for analyzing the mechanical behavior of nanowires in atmosphere compared with the model in vacuum. Furthermore, it is possible to use this characterization method to measure the size-related elastic properties of similar wire-sharp nanomaterials in atmosphere and estimate the corresponding mechanical behavior. The study of the size-dependent Young's modulus in ZnO nanowires in atmosphere will improve the understanding of the mechanical properties of nanomaterials as well as providing guidance for applications in NEMSs, nanogenerators, biosensors and other related areas.

Study on the interface electronic states of chemically modified ZnO nanowires

In this work, ZnO nanowires were modified with three mercaptans.

Energy level alignment in TiO2/metal sulfide/polymer interfaces for solar cell applications

Semiconductor sensitized solar cell interfaces have been studied with photoelectron spectroscopy to understand the interfacial electronic structures. In particular, the experimental energy level alignment has been determined for complete TiO2/metal sulfide/polymer interfaces. For the metal sulfides CdS, Sb2S3 and Bi2S3 deposited from single source metal xanthate precursors, it was shown that both driving forces for electron injection into TiO2 and hole transfer to the polymer decrease for narrower bandgaps. The energy level alignment results were used in the discussion of the function of solar cells with the same metal sulfides as light absorbers. For example Sb2S3 showed the most favourable energy level alignment with 0.3 eV driving force for electron injection and 0.4 eV driving force for hole transfer and also the most efficient solar cells due to high photocurrent generation. The energy level alignment of the TiO2/Bi2S3 interface on the other hand showed no driving force for electron injection to TiO2, and the performance of the corresponding solar cell was very low.

Zn K edge and O K edge x-ray absorption spectra of ZnO surfaces: implications for nanorods

Zn K edge and O K edge x-ray absorption near-edge structure (XANES) spectra of ZnO surfaces are calculated. The difference between theoretical XANES for ZnO surfaces and ZnO bulk is then compared to the earlier observed differences between experimental XANES for ZnO nanostructures and ZnO bulk as taken from the literature. It follows from our calculations that the differences between the experimental XANES of bulk ZnO and nanocrystalline ZnO is not due to the enhanced role of the surfaces in nanostructures. Rather, the difference in XANES has to reflect differences in the local geometry around the photoabsorbing sites. The dependence of XANES of ZnO surfaces on the polarization of the incoming radiation is also investigated theoretically and found to be similar as in the bulk.

Electronic excitation energies of Zn(i)S(i) nanoparticles

The excitation energies of small ZnS nanoclusters characterized in previous studies have been calculated using TDDFT. The relativistic pseudopotentials of Stevens et al have been used, including Zn 4s(2) electrons and S 3s(2) and 3p(4) electrons as valence electrons. Results obtained with these pseudopotentials are compared to those obtained considering also Zn 3s(2)3p(6)3d(10) electrons in the valence part, and demonstrated to be consistent. The results show that spheroid-like bubble structures have absorption energies in the range of 5-5.3 eV for small sizes, which decreases to 5 eV with increasing particle size.

Size dependence of Young's modulus in ZnO nanowires

We report a size dependence of Young's modulus in [0001] oriented ZnO nanowires (NWs) with diameters ranging from 17 to 550 nm for the first time. The measured modulus for NWs with diameters smaller than about 120 nm is increasing dramatically with the decreasing diameters, and is significantly higher than that of the larger ones whose modulus tends to that of bulk ZnO. A core-shell composite NW model in terms of the surface stiffening effect correlated with significant bond length contractions occurred near the {1010} free surfaces (which extend several layers deep into the bulk and fade off slowly) is proposed to explore the origin of the size dependence, and present experimental result is well explained. Furthermore, it is possible to estimate the size-related elastic properties of GaN nanotubes and relative nanostructures by using this model.

Water adsorption on ZnO(101̄0): from single molecules to partially dissociated monolayers

Static and dynamic density functional calculations have been used to study the structure and energetics of water adsorbed on the main cleavage plane of ZnO. In the single molecule limit we find that molecular adsorption is strongly preferred. The water binding energy increases for higher coverages due to an almost isotropic attractive water-water interaction which leads to clustering and formation of monolayer islands in the low water coverage regime. A thermodynamic analysis further shows that the full water monolayer is clearly the most stable phase until water starts to desorb. The water monolayer is even more stabilized by a partial dissociation of the water molecules, yielding as most stable configuration a (2x1) superstructure where every second water molecule is cleaved. The dissociation barrier for this process is very small which allows for an auto-dissociation of the water molecules even at low temperatures as observed experimentally. Finally we find that the energy cost involved to form [1210]-oriented domain boundaries between (2x1) patches with different orientation is almost negligible which explains the abundance of such domain boundaries in STM images.

Adsorption of atomic hydrogen on ZnO(101̄0): STM study

The adsorption of atomic hydrogen on a single crystal ZnO(1010) surface has been studied by scanning tunneling microscopy (STM) under ultrahigh vacuum conditions at room temperature and at elevated temperatures. High resolution STM images indicate that a well-ordered (1x1) H adlayer is formed on the ZnO(1010) surface. The STM data strongly indicate that the hydrogen adsorbs on top of the oxygen atoms forming hydroxyl species. Scanning tunneling spectroscopy (STS) studies reveal a H atom induced metallization at room temperature. In contrast to the clean surface for the hydrogen-covered surface distinct defects structures consisting of missing O and Zn atoms could be identified.

Electronic excitation energies of Zn(i)O(i) clusters

Time-dependent density-functional theory (TDDFT) is used to study the excitation energies of the global minima of small Zn(i)O(i) clusters, i = 1-15. The relativistic compact effective core potentials and shared-exponent basis set of Stevens, Krauss, Basch, and Jasien (SKBJ), systematically enlarged with extra functions, were used throughout this work. In general, the calculated excitations occur from the nonbonding p orbitals of oxygen. These orbitals are perpendicular to the molecular plane in the case of the rings and normal to the spheroid surface for 3D clusters. The calculated excitation energies are larger for ringlike clusters as compared to 3D clusters, with the excitation energies of the latter structures lying close to the visible spectrum. The difference between Kohn-Sham eigenvalues of the orbitals involved in the electronic excitations studied have also been compared to the TDDFT results of the corresponding excitations for two approximate density functionals, that is, MPW1PW91 and B3LYP, the latter being more accurate. Moreover, they approach the TDDFT value as the cluster size increases. Therefore, this might be a practical method for estimating excitation energies of large Zn(i)O(i) clusters.